1. Field of the Invention
The present invention relates to a radioelectric antenna which enables to configure in space one or several lobes or beams, these terms being here equivalent, for transmitting/receiving electromagnetic waves and hence to configure its radiating diagram. It finds applications in the domain of the transmission/reception in radio electromagnetic waves and in particular as an antenna for mobile telephony. It enables in particular the shaping and the commutation of radioelectric beams or lobes within a base transceiver station of a telephony network or radiocommunication data transmission network with mobile stations as well in transmission as in reception (E/R).
2. Description of Related Art
Generally, to manufacture a directable shaped-beam antenna, there is provided on the one hand a structurally shaped-beam antenna and, on the other hand, it is moved in order to be directed in space, general in rotation, so that its electromagnetic radiation diagram is directed according to the direction requested. On top of the fact that the mechanical displacement of the antenna requires mechanical means which may be costly, are subjected to wear and are complex to be maintained, the antennas being generally in high locations and in severe weather conditions, the radiation diagram remains identical in its form throughout the rotation.
It is hence desirable to have non-mechanical means enabling to modify the orientation of the radiation diagram in space. Moreover, it also appears desirable to be able to modify the structure of the radiation diagram, in particular the number of transmission/reception lobes and/or their forms in space.
Indeed, for example in the case of new broadband radiocommunication services, it appears that only the dynamic systems fitted with smart antennas will enable optimal usage of the Hertzian spectrum while employing adaptation capacities of transmission/reception spatial configuration as shown in the articles “The path towards UMTS—Technologies for the information society”, UMTS Forum 1998 and, “Une vue globale du concept d'UMTS” (a global view of the UMTS concept), S. Breyer, G. Dega, V. Kumar and L. Szabo, of Alcatel.
These smart antennas offer the possibility of increasing the capacity of the systems operating in particular in CDMA (“Code-Division-Multiple-Access”) mode thanks to the use of a pseudo-SDMA (“Spatial-Division-Multiple Access”) technique according to modalities known as described in the article “Smart antennas enhance cellular/PCS performance, part 1 & 2”, C. B. Dietrich Jr. and W L Stuztman, in Microwaves & RF, April 1997. This technique enables to reduce the “co-channel” interferences in the downlink (base transceiver station towards the mobile phone) of the cellular networks in forming a shaped beam directed towards the mobile phone. It also enables rejection of the interferences in the uplink (mobile phone towards station base) with additionally the possibility of forming the diagram of the antenna of the base transceiver station so that it exhibits a reception valley in the direction of the interferences.
Generally two categories of so-called smart antennas can be distinguished and which have a variable radiation diagram: those made with beam-switching antenna networks and those made with adaptative antennas as presented in “Experiments on adaptative array diversity transceiver for base transceiver station application in W-CDMA mobile radio” par M. Sawahashi and S. Tanaka during the AP-S 2000, Salt Lake City, USA, July 2000.
The smart antennas made with adaptative antennas are generally constituted of a network of radiating elements controlled by a digital signal processor (DSP). They may adapt automatically their radiation diagram relative to the external signals received.
Unfortunately, the current digital technology does not appear mature enough for supporting the multiple frequency bands necessary in the mobile telephony, as well as the powers necessary for mastering this radio spectrum. Moreover, the technology of the smart and digital adaptative antennas is not very adapted to the existing technology in the base transceiver station BTS and hence would require too large investments to renew these as noticed in the presentation of M. Sawahashi quoted previously.
The smart antennas made with beam-switching antennas use the analogue synthesis of multiple beams. This approach keeps most features of the digital smart antennas, with however much smaller complexity and cost. It is compatible with the existing infrastructures (in particular the base transceiver stations) and enables significant increase in the capacity of the network with respect to the investment. Traditionally, the beam-switching antennas use a supply network with pre-set phase which provides several output ports corresponding, each, to a beam of fixed direction. Base transceiver stations of this type have been tried out by numerous companies in the United States and in Europe, in particular by: Celwave associated with BellSouth, Hazeltine Corp., Metawave Communications, ArrayConun Inc., Ericsson, Nortel, . . . . In addition to the articles and to the presentation quoted previously, information on that subject is also available in “Novel multiple-beam antenna array serves mobile BTS, part 1”, L. Cellai and A. Ferrarotti, Microwaves & RF, August 1999 or in “Array antenna design for base transceiver station applications”, B. Johannisson and A. Derneryd, Ericsson Microwave Systems AB.
The main shortcoming of this beam-switching technology is the great number of radiating elements and hence its cost. It has hence been suggested using an alternative solution to manufacture beam-switching type antennas while placing a passive radiating element at the core of a set of rods made of Photonic Band Gap (PBG) material, certain of these rods being rendered active by the insertion of switching components enabling, by an appropriate control, to force the rods to behave like discontinuous rods and for others such as continuous rods which exhibit different radioelectric features of the former. Information on this subject is available in the presentation “Beam switching smart antenna for hyperlan terminals” by A. Chelouah, A. Sibille, C. Roblin, during AP2000, Davos, April 2000, or, still, in the article of E. Yablonovitch in Physical Review Letters, vol. 58, n'20, 1987, p2059-2062.
This alternative solution does not involve any direct action on the excitation circuit of the radiating element but only on elements of its close environment, thereby limiting the losses. It is obtained by using the properties of the Photonic Band Gap (PBG) materials which are already known and for which articles have been published, in particular: “Photonic Band Gaps in experimentally realisable periodic dielectric structures”, C. T. Chan, K. M. Ho and C. M. Soukoulis, Europhysics Letters, 16(6), pp 563-568, 7 Oct. 1991; or “Metallic Photonic band-gap materials”, M. M. Sigalas, C. T. Chan, K. M. Ho and C. M. Soukoulis, Physical Review B, vol. 52, n'16, 15 Oct. 1995; or, finally, “Active Metallic Photonic B and Gap materials (MPBG): experimental results on beam shaper”, G. Poilasne, P. Pouliguen, K. Mahdjoubi, L. Desclos and C. Terret, IEEE Trans. on Antennas and Propagation, January 1999.
The assembly of the rods forming the PBG material of this type of antenna is a periodic structure, so-called PBG structure, composed mainly of parallel conductors and wherein a radiating element acts. The electromagnetic features of this PBG structure depend mainly on the transmission/reception frequency of the radiating element. Its frequency response at a planar wave exhibits alternately frequency bands authorising or not propagation through the PBG structure. The response duality between a PBG material composed of continuous rods and a PBG material composed of discontinuous rods has been studied. These differences have been exploited for obtaining the switching and the spatial shaping of the radiation diagram by passing from one to another, continuous or discontinuous rods, of these PBG structures. It is thus that the presentations and articles have been produced in this field as in particular: “Numerical and experimental demonstration of an electronically controllable PBG in the frequency range 0 to 20 GHz”, par A. De Lustrac, T. Brillat, F. Gadot, E. Akmansoy, during AP2000, Davos, avril 2000; and in “Experimental radiation pattern of dipole inside metallic photonic band-gap materials”, of G. Poilasne, P. Pouliguen, K. MahdJoubi, C. Terret, P. Gelin and L. Desclos, in Microwave and Optical Technology Letters, vol. 22, Issue 1, July 1999.
Currently, the PBG structures with square meshed are used. In other words, and as illustrated on FIG. 1 (cross-section with respect to the axis of the rods), the rods 1 constitute a square-mesh grid at the centre of which the passive radiating element 2 is situated.
It appears that this PBG material with square meshes exhibit two major shortcomings. First of all, it is ill-suited to excitations by cylindrical waves, hence a difficult study when a radiating element is placed at the centre of a PBG material with square meshes. Besides, it does not enable to create a constant beam rotating round 3600 with any pitch and any angle.